The present invention relates to cellular communications and, more particularly, to a method for tracking mobile users in a cellular network.
The increasing demand for personal communication services (PCS) requires wireless networks to gracefully accommodate mobility of both users and services. Contrary to wired networks, in which user location is fixed, in wireless networks a user can potentially be located anywhere within the system service area. As the number of mobile users keeps increasing, the amount of signaling traffic required for location management keeps growing. The cost associated with the need to locate a mobile user is composed of two parts: 1) The cost of accessing data bases, such as Home Location Register (HLR) and Visitor Location Register (VLR), and 2) The cost of radio signaling over the control channel.
The problem of tracking mobile users has been addressed by several studies, many of which attempt to reduce the wireless cost of user tracking. In Rose C., "Minimizing the Average Cost of Paging and Registration: A Timer-Based Method", ACM Journal of Wireless Networks, Vol. 2, No. 2, pp. 109-116 (1996), a timer based method was suggested in which the user updates its location every T time units, where T is a time threshold. Each time the user makes no contact with the network for T units of time, the user initiates a registration message. Another strategy, suggested in Bar-Noy A., I. Kessler and M. Sidi, "Mobile Users: to Update or not to Update?", Wireless Networks, Vol. 1, No. 2, pp. 175-185 (1995), and in Madhow U., L. Honig and K. Steiglitz, "Optimization of Wireless Resources for Personal Communications Mobility Tracking", IEEE Trans. on Networking, Vol. 3, No. 6, pp. 698-707 (1995), is to use a distance-based method in which the user tracks the distance it moved from its last known location, in terms of cells. Whenever this distance exceeds a parameter D the user transmits a registration message. The distance-based strategy is considered as the most efficient tracking strategy, however its implementation is the most difficult. In Bar-Noy et al. (1995), and in Akyildiz, I. F., J. S. M. Ho and Y. B. Lin, "Movement Based Location Update and Selective Paging Schemes", IEEE/ACM Trans. on Networking, Vol. 4, No. 4, pp. 629-638 (1996), a movement based method was suggested, in which the user counts the number of cell transitions, and transmits a registration message whenever this number exceeds a pre-defined threshold. A load-sensitive approach, in which the tracking activity adapts to both user and system activity was suggested in Levy H. and Naor Z., "Initiated Queries and their Application for Tracking Users in Wireless Networks", Sixth WINLAB Workshop, 1997, pp. 381-399, and in Naor Z. and Levy H., "Minimizing the Wireless Cost of Tracking Mobile Users: An Adaptive Threshold Scheme", IEEE INFOCOM'98 pp. 720-727. The basic assumption underlying all these methods is that the user location, as well as other derived parameters (such as the distance traveled from its last known location), are always known to the user. However, in reality, this information is not available to the user.
Existing cellular systems use the following tracking strategy, known as the geographic-based (GB) strategy: The geographic area is partitioned into location areas, based on the commercial licenses granted to the operating companies. A location area (LA) is a group of cells, referred to as a home-system. The term location area is used by GSM systems, while IS-41 refers to the LA as registration area. Users register whenever they change LA, while within the LA they never register. The implementation of the geographic-based (GB) strategy is very simple: All base stations within the same LA periodically broadcast a location area code wherein is encoded the ID of the LA. Each user receives the location area code of the LA wherein it is located, compares its last LA ID with the current ID, and transmits a registration message whenever the ID changes. Hence, the user is not aware of its exact location within the LA. When there is an incoming call directed to a user, all the cells within its current LA are paged. Because the number of cells within a typical LA is very large, the tracking cost associated with the GB strategy is very high.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method for tracking users of a cellular communications network that is based on user location measured at the cell level.